Energy saving in electric appliances is an unavoidable important problem that has been addressed in recent years. Also, in a heat insulation box used for a refrigerator and other various kinds of electric appliances, improvement in performance of a heat insulator is becoming essential. On the other hand, positive efforts to conserve terrestrial environment is becoming important. One of urgent requests with regard to electric appliances is energy saving; thus, improving heat insulating properties of heat-related electric apparatus is becoming an important problem to address.
One heat insulator that has been recently developed, mainly by manufacturers of electric appliances and heat insulators for energy and space saving, is a vacuum heat insulator that has excellent heat insulating properties. An example of the vacuum heat insulator is made by covering a core, made of a rigid urethane foam having continuous pores with a gas-barrier laminated film, and evacuating an inside thereof. This vacuum heat insulator has heat insulating properties that are approximately 2.5 times heat insulating properties of conventional rigid or soft urethane foam or resin foam.
Japanese Patent Examined Publication No. H05-63715 discloses a vacuum heat insulator using a fibrous aggregate. A use of a fibrous aggregate of glass fibers, ceramic fibers, or resin fibers as a core of a vacuum heat insulator provides a light and deformable vacuum heat insulator.
Moreover, according to Japanese Patent Examined Publication No. 30-3139, a vacuum heat insulator made of a core of glass fibers, each having a diameter of 250 μm or smaller, is proposed. Inside of the vacuum heat insulator is maintained to a degree of vacuum of 0.75 Pa or lower. Japanese Patent Laid-Open Publication No. 60-208226 discloses randomly laminated inorganic fibers, having a small diameter, in a direction perpendicular to a heat transfer direction, and other fibers are sewn perpendicularly halfway to the laminated inorganic fibers to form a core of a vacuum heat insulator.
An example of binding fibers using a binder is disclosed in Japanese Patent Laid-Open Publication No. H09-138058. In this invention, a fiber material such as glass wool is molded using an organic binder, and used as a core of a vacuum heat insulator.
However, these conventional techniques have following problems and thus are difficult to be put to practical use.
For example, the vacuum heat insulator disclosed in Japanese Patent Examined Publication No. 30-3139 is difficult to be formed into a specific shape because it is made of glass fibers only. When a sheet-form vacuum heat insulator is to be produced, using glass fibers as a core of the vacuum heat insulator requires much manpower because the fibers themselves do not have shape-keeping properties.
Since inorganic fibers are sewn with other fibers in Japanese Patent Laid-Open Publication No. 60-208226, shape-keeping properties are imparted to the fibers themselves, and the problem with regard to shape-keeping properties is solved. However, as general methods cannot be used to sew the fibers, while reducing heat conduction, this process has a problem of high production costs.
Japanese Patent Laid-Open Publication No. H09-138058 proposes binding fibers together using an organic binder as a method of imparting shape-keeping properties to the fibers. However, this publication only discloses a type of the binder and does not disclose an amount of the binder or a composition of the fibers. Thus, there is a problem in that it is difficult to bind fibers together using the binder while maintaining heat insulating properties suitable for a vacuum heat insulator. In addition, when organic fibers are used for a core, the core generates gases during a long-term usage, and thus, heat insulating properties may be degraded.
In order to improve heat insulating properties of a heat insulation box, a heat insulation box that uses a heat insulator using a resin foam or powder as the core has been proposed. Such a core has a problem of long-term heat insulating properties or workability. As described above, conventional techniques have problems such as poor workability of the vacuum heat insulator, or a premature stage of product development, and advantages of fiber aggregates are not utilized sufficiently.
In consideration of the above problems, the present invention aims to provide a heat insulation box excellent in heat insulating properties and in productivity by using a vacuum heat insulator that includes a core made of laminated sheets of inorganic fibers. The core of the vacuum heat insulator is excellent in long-term reliability and in workability.